In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an imagewise way on a receiving member substrate, the latter not bearing any imagewise latent electrostatic image. The substrate can be an intermediate endless flexible belt (e.g. aluminium etc.). In that case the imagewise deposited toner must be transferred onto another final substrate. Preferentially the toner is deposited directly on the final receiving member substrate, thus offering a possibility to create directly the image on the final receiving member substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
A DEP device is disclosed by Pressman in U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising
a layer of insulating material, called isolation layer; PA1 a shield print electrode consisting of a continuous layer of conductive material on one side of the isolation layer;; PA1 a plurality of control print electrodes formed by a segmented layer of conductive material on the other side of the isolation layer; and PA1 at least one row of apertures. PA1 which comprises different isolated wires as control back electrode PA1 and the toner delivery unit. For a line printer the density can be tuned by selecting an appropriate voltage for shield print electrode, control print electrode and control back electrode wire. PA1 a printhead structure, at the front side of the receiving member substrate, having a plurality of apertures each with one galvanically isolated control print electrode; PA1 a toner delivery means, at the front side of said printhead structure, providing toner particles in the vicinity of said apertures; and PA1 a support for the back side of the receiving member substrate, having a plurality of galvanically isolated control back electrodes;
Each control print electrode is formed around one aperture and is isolated from each other control print electrode.
Selected potentials are applied to each of the control print electrodes while a fixed potential is applied to the shield print electrode. An overall applied propulsion field between a toner delivery means and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control print electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield print electrode may face the toner delivery means and the control print electrode may face the receiving member substrate. A DC field is applied between the printhead structure and a single shield back electrode on the receiving member support. This propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the shield back electrode.
This kind of printing engine, however, requires a rather high voltage source and expensive electronics for changing the overall density between maximum and minimum density, making the apparatus complex and expensive.
To overcome this problem several modifications have been proposed in the literature.
In U.S. Pat. No. 4,912,489 the conventional positional order of shield print electrode and the control print electrode--as described by Pressman--has been reversed. This results in lower voltages needed for tuning the printing density. In a preferred embodiment, this patent discloses a new printhead structure in which the toner particles from the toner delivery means first enter the printhead structure via larger apertures, surrounded by so-called screening electrodes, further pass via smaller apertures, surrounded by control print electrodes and leave the structure via a shield print electrode. The larger aperture diameter is advised in order to overcome problems concerning crosstalk.
In EP-A-0 587 366 an apparatus is described in which the distance between printhead structure and toner delivery means is made very small by using a scratching contact. As a result, the voltage--needed to overcome the applied propulsion field--is very small. The scratching contact, however, strongly demands a very abrasion resistant top layer on the printhead structure.
An apparatus working at very close distance between the printhead structure and the toner delivery means is also described in U.S. Pat. No. 5.281,982. Here a fixed but very small gap is created in a rigid configuration making it possible to use a rather low voltage to select wanted packets of toner particles. However, the rigid configuration requires special electrodes in the printhead structure and circuits to provide toner migration via travelling waves.
On the other hand it has been known for a long time that systems of the type "contrography" can be used to select toner particles according to an image pattern. In U.S. Pat. No. 4,568,955 e.g. a segmented receiving member support comprising different galvanically isolated styli as control back electrodes is used in combination with toner particles that are migrated with travelling electrostatic waves. The main drawback of this apparatus is its limited resolution and dependence of the image quality on environmental conditions and properties of the receiving member substrate.
In U.S. Pat. No. 4,733,256 some of these drawbacks are overcome by the introduction of a printhead structure, as described by Pressman. The printhead structure is located between the receiving member support
In U.S. Pat. No. 5,036,341 a device is described comprising a screen or lattice shaped control back electrode matrix as segmented receiving member support. This apparatus has the advantage that matrix-wide image information can be written to the receiving member substrate, but it also suffers from the environmental influences and those caused by the nature of the receiving member substrate.
To overcome these drawbacks Array Printers described in U.S. Pat. No. 5,121,144 another device wherein the segmented back electrode without printhead structure was changed into a two part electrode system, having a printhead electrode structure and a back electrode structure. A first part was placed between the toner delivery means and the receiving member substrate and consisted of parallel, isolated wires, being used as printhead structure. A second part consisted of another set of parallel wires, arranged orthogonally with respect to the first wires and was used as back electrode structure. The receiving member support or back electrode structure in all examples consists of isolated wires which are oriented in one direction. As printhead structure, there are described three different configurations:
1. isolated wires in a cross direction; PA0 2. a flexible PCB with only control print electrodes in the cross direction; and PA0 3. a flexible PCB with common shield print electrode and control print electrodes in the cross direction.
The different systems according to this patent make it possible to change the propulsion field in a group of apertures, tuning the density by setting the voltage of the different control print electrodes.
All the patents or applications mentioned above make the experimental configuration of the DEP-device much more complicated. On the other hand it would be very advantageous to have an apparatus with less complicated parts, being operative with very small voltages.
There is thus still a need to have a system for practising DEP, that--while avoiding the problems cited above--is based on a simpler structure, yielding high quality images in a reproducible and constant way.